System and method for cooling and extraction of heavy ashes with increase in total boiler efficiency

ABSTRACT

A cooling system for heavy ashes of the type adapted to be used in association with a combustion chamber, in particular for large flow rates of heavy ashes deriving, for example from solid fossil fuel, in an energy-production unit is described. The cooling system has: a transport belt for transporting the heavy ashes, adapted to be arranged below the combustion chamber and having a containment casing and a transport surface equipped with openings for the transit of cooling air, and cooling means for cooling the heavy ashes received on the transport surface.

FIELD OF THE INVENTION

The present invention relates to a cooling system for heavy ashes of thetype apt to be used in association with a combustion chamber, inparticular for large flow rates of ashes deriving for example from solidfossil fuel in an energy-production unit.

BACKGROUND OF THE INVENTION

Well-known dry cooling and extraction systems for heavy ashes producedin combustion chambers (or boilers) from solid fuel are based upon aircooling of an ash bed. To this end, the latter is transported on a beltresistant to high temperatures placed just below the throat of theboiler. Cooling air is drawn into the extraction system by the negativepressure present within the boiler, passing through proper openings of acontainment casing of the belt. The air then runs the system and the ashbed in countercurrent to the sense of direction, thus operating thecooling of ashes and equipments.

A system for extraction and cooling of that type just described isdisclosed in EP 0 252 967.

The above-mentioned mechanism of heat exchange between ashes and air incountercurrent affects the size of the extraction system, in terms of:

-   -   air flow rate used, where the speed of air itself within the        system has to be maximized to increase, as a result, the heat        exchange coefficient with the ash; and    -   wheelbase and speed of transport belt, parameters that are to be        optimized to increase the residence time of the ash in contact        with the air and to limit the height of ash layer.

In fact, the efficiency of ash cooling is limited to the exposed andavailable surface for the heat exchange with the air. In particular,given the isolating nature of ash, the first layers licked by the airget cool whereas the inner layers of the ash remain at temperature.

Therefore, a perfectible first aspect of the known system is related tothe heat exchange mode between ash and cooling air.

The relationship between the amount of cooling air and ash is typically3:1, i.e. 3 tons of air are needed to cool 1 ton of heavy ashes.However, since, downstream of the ash cooling, all the air introducedinto the boiler is drawn from the bottom of this, the amount of coolingair must not exceed 0.5 to 2, 0% of the total air of combustion. Infact, if it is alter over such limit, the stechiometric ratio betweenfuel and air results in a reduction of combustion efficiency and in anincrease of losses to the fireplace.

In particular, in the known systems mentioned above, factors thatcontributes to the increase in combustion efficiency (positive terms)are:

-   -   the chemical energy of the, recovered by sensitive heat of        cooling air thanks to the post combustion of unburned on the        extracting belt favoured by the air;    -   the sensitive ash heat, recovered through the sensitive heat of        the cooling air reintroduced into the boiler; and    -   the recovery of radiant floe at the throat of the boiler,        absorbed by irradiated components of the system and transferred        to the cooling air and extracted ashes.

The factor that instead determines a decrease of the efficiency of theboiler (negative term) is the loss of efficiency at the air/smokepre-heater. The latter involves the use of ambient air which preciselycools the combustion smokes, by pre-heating them. Such preheated air issent into the combustion chamber. However, this specific amount of airhas to be reduced to take account of the air introduced from the bottomof the boiler, and thus the lower intake of cooling air in thepre-heater determines higher temperatures of output fumes from thelatter.

Therefore, from the point of view of the overall combustion efficiencythe system of heat exchange between ash and air cooling is perfectible.

It should also be noted that the continued growth in demand for solidfossil fuels for the production of electric energy makes it even morefrequent also the combustion of coals or lignites with high ash grade.The combustion of these latter in high power boilers leads to asignificant production of heavy ash, even up to 100 tons/hour, oftencontaining high percentages of unburned. The dry cooling or mostly dryof these quantities requires considerable flow of cooling air, even twoor three times greater than fossil fuels with high calorific value.

This leads to the important drawback that, in the prior art systemsconsidered above, the amount of air required for cooling of ashes ismuch higher than the maximum percentage re-introduced into thecombustion chamber through the throat of the boiler.

SUMMARY OF THE INVENTION

According to the explanations in the previous section, the technicalproblem posed and solved by the present invention is to provide a systemand a method that are optimized in terms of heat exchange betweenextracted ashes and air cooling, and that allow to overcome thedrawbacks mentioned with reference to the prior art.

This problem is solved by a system according to claim 1 and by a methodaccording to claim 17.

Preferred features of the present invention are present in the dependentclaims thereof.

As explained in more detail below, the invention provides that aninsufflating system—i.e. forced feeding system—is associated to the mainextractor belt connected to the throat of the boiler—that takes theambient air and pushes it inside the containing casing of extractor, incorrespondence of one or more partitioned regions below the transportsurface of the belt. The latter has dedicated openings—typically holesor cracks in the form of millings—which allows the passage of coolingair through it and then through the ash layer.

In this configuration, the partitioned that allows directing the coolingair through the holes-cracks, minimizing air outflows of the regions ofinterest. Therefore, when the cooling air crosses the bed of ashrealizes with it a heat exchange of called “cross flow” type,characterized by a heat exchange efficiency much superior to that of thedry extraction systems already known, and that due to the increased areasurface of ash affected to the heat transfer. The cooling air is thenintroduced into the boiler from the bottom of this.

In a preferred configuration, any outflows of cooling air, coming outfrom lights present between the transport surface and said partitionedregion below without crossing the bed of ash, are recycled to thepartitioned region of the main extractor, preferably by means of thesame insufflating system.

Thus, the invention allows to maximize the efficiency of heat exchangebetween the ash on the extracting belt and air cooling and so thecooling efficiency of the ashes.

This implies a strong reduction on the ratio between the quantity ofcooling air and flow of extracted ash and therefore the possibility tominimize the amount of cooling air reintroduced into the combustionchamber from the bottom, with a consequent strong reduction in lossesassociated with the air/smoke exchanger and an increase in net overallefficiency of combustion.

In addition, the configuration of the invention, particularly in thepreferred embodiments illustrated below, allows making the extractionand cooling system extremely compact and simple, even for thepossibility of eliminating a auxiliary wet cooling system present insome known systems.

In a particularly preferred configuration, there is also a further stageof cooling, preferably located downstream of a stage of crushing andbased on an auxiliary heat exchanger of tube bundle type crossed by aworking fluid cooling.

In said further stage of cooling is expected a fluidization of the ashcontained in a cooling volume, preferably performed by the same ambientair recovered for cooling on the main extraction belt. This fluidizationallows an improved heat exchange with the surfaces of the tube sheaf.The head ash to be fluidized can be obtained by making a transport beltplaced downstream the cooler and similar to the main extracting beltworking in an extraction manner.

According to another preferred feature of the invention, the mainextracting belt connected to the throat of the boiler by means of ahopper selectively closable to block the ash flow. During theaccumulation stage of the ash on the closed bottom of the hopper, thereis a cooling of the ash by the same forced feeding system of coolingair, preferably obtained by the same main forced feeding systemassociated with the cooling on the belt.

It will be appreciated that said cooling system allows avoiding animportant drawback of the known systems, namely the fact that at theopening of the hopper, large amounts of ash at high temperature have tobe cooled on the belt, potentially out of the nominal parameters of thesystem.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages, features and conditions of employment of the presentinvention will be apparent from the following detailed description ofcertain preferred embodiments presented for illustrative purpose withoutlimiting the scope of the invention. Reference will be made to thefigures of the annexed drawings, wherein:

FIG. 1 shows a schematic representation in side view of an extraction,cooling and transporting system of ashes according to a preferredembodiment of the invention;

FIG. 2 shows a schematic cross-sectional view of the system of FIG. 1,performed along the line AA of the latter and suitable to highlight afirst preferred embodiment of a partitioned region of said system;

FIG. 2A shows a schematic perspective view of part of the regionpartitioned by lateral baffles of FIG. 2;

FIG. 3 shows a schematic cross-sectional view of the system of FIG. 1,performed along the line AA of the latter and suitable to highlightsecond preferred embodiment of a partitioned region of the system;

FIG. 3A shows a magnified view of a detail of the partitioned region ofFIG. 3;

FIG. 4 shows a schematic magnified view of a detail of the system ofFIG. 1, showing the presence of additional cooling stages of the ashes;

FIG. 5 shows a cross section view of the system of FIG. 1 performed atthe throat of the boiler, showing a cooling circuit of the ash in thehopper;

FIG. 6 shows a plan view of a detail of the transport belt, showing thepassage openings for a cooling air flow, and

FIG. 6A shows a cross section view of the belt of FIG. 6, performedalong the line AA of the latter figure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIG. 1, a preferred embodiment of extraction andcooling system of heavy ashes of the invention is globally indicatedwith 1. The system 1 is of the type apt to be used in association with acombustion chamber or boiler 2, in particular for large flow rates ofashes deriving for example from solid fossil fuel in anenergy-production unit.

The boiler 2 may be an integral part of the system 1 or providedseparately from it and is equipped with an extraction hopper 21, thelatter typically lined inside with refractory material. The hopper 21 isassociated with a system that allows the closing of its bottom and thenof the boiler 2 throat, which will be described in greater detail later.

The initial part of a continuous transport belt 31, moving along aclosed path, is placed in correspondence of the bottom of the boiler 2.During the use, the belt 31 receives from the hopper 21 the ashesproduced by the boiler 2 and substantially carries them in the form ofcontinuous bed. In particular, the ashes are received on a uppertransport surface 311 of the belt 31 during a return run of it. On thistransport surface 311, during the movement away of the ashes from theboiler bottom 2, takes place the dry cooling of the ashes themselves, bymeans of a flow ambient air which is sent into a containment casingsystem 3 of the belt 31 according to ways that will be shortlydescribed.

The transport belt 31 and its casing 3 may have a globally constructionof the kind described in EP 0 252 967 or EP 0 931 981.

Moreover, as shown in FIGS. 6 and 6A, passing openings for cooling air 9are made on the continuous belt 31 for example in the form of holes or,as represented, cracks obtained by milling.

Always with reference to FIG. 1, the system 1 is equipped with means ofcooling of the ashes received on the transport belt 31, apt to determinea feeding of cooling air in correspondence of those ashes.

Such cooling means include means of forced feeding of air, for examplebased on a blower or compressor 11 and on an associated intake pipe ofambient air 111, the latter preferably equipped with appropriate controlmeans selectively operable, in particular a valve 112. Such aspiratedambient air is sent to a feeding pipe, globally denoted by 13, thatconduct it to a partitioned region 4 associated with the belt 31. Eventhe feeding to the partitioned region 4 is preferably controlled byappropriate control means selectively operable, in this case inparticular a valve 134.

In FIG. 1, a single partitioned region 4 was represented for simplicity,arranged downstream of the bottom boiler with respect to the directionof advancement of the transport surface 311 and below to the latter,interposed between the there and back tract of the belt 31. However, thepartitioning preferably extends longitudinally to cover the whole bottomof the transport surface 311 (as shown in FIGS. 2, 2A and 3, 3A).

Moreover, in one embodiment, different partitioned regions may be madediscretely distributed along said transport surface 311, below it.

The partitioned region 4 is apt to minimize outflows of alleged coolingair in it, so that such air passes almost entirely through the openings9 of the transport belt 31, thereby effectively cooling the bed of ashesreceived on the transport surface 311.

The blower or compressor 11 then generates a suitable pressure gradientto overcome the losses distributed and concentrated along the circuit 13and associated with the transport belt 31 and the overlooking layer ofash.

In a first embodiment shown in FIGS. 2 and 2A, the partitioned region 4is affected by transverse baffles 6, arranged transversely to thetransport surface 311 with respect to the direction of this advancement,and bounded laterally by two longitudinal baffles 7, spanning accordingsaid progress direction.

The lateral baffles 7 are arranged near the transport surface 311 andits roller support 14, so as not to interfere with the movement of eachbody but at the same time minimizing light outflows through the aircooling alleged in the partitioned region 4.

In addition, the arrangement of the transverse baffles 6 of thepartitioning of the region below the transport surface 311 ensures alabyrinth seal to the cooling air, assisting the sealing action to thelateral air outflows by means of baffles 7.

Always in this example, the partitioned region 4 is bounded below by atilted-surface plate 5 for recovering of any lost fines during thetransport on the surface 311. The longitudinal baffles 7 have each acorresponding lower end door 72, selectively openable to the outsidethrough a mechanism 71, preferably hinged, for the downflow of finestoward the bottom of the containment casing 3 (where they can berecovered by a cleaning system not shown). Preferably, the downflowsystem based on door items 72—mechanisms 71 is timed.

In a second embodiment of the partitioned region 4 shown in FIGS. 3, 3A,transverse baffles 6 are still provided, in this case associated withbulkheads or lateral baffles 51 which extend longitudinally along thebelt 31, substantially parallel to it, upper to the transport surface311 and each along a respective side of this, where the contact orproximity of said baffles 51 with the containment ends 81 of thetransport belt 31, allows limiting the passage of air that does not passthrough the hole belt 31.

In this second embodiment, each of the tilted-surface plate 5 presents alower end door 725 selectively openable to the outside through amechanism 715, preferably hinged, for the purposes of downflow of finestoward the bottom of said containment casing 3. Thus, when—during normaloperation—the door 725 is closed, it keeps a tight air, being in directcontact with the side wall of the casing 3.

A further embodiment may provide the combined presence of these sidewalls arranged at the upper transport surface, of the lateral bafflesplaced lower to the latter and with selectively openable doors and of atile, the latter typically without doors as in the first embodimentdescribed above.

The globally configuration of the cooling means is such that the ashlayer transported on the surface 311 is cooled by a flow of ambient airpassing through it transversely from the bottom to the top along theentirely length of the cooling forced region consists of the partitionedregion 4 and comprises between the first and the last transverse baffle6. The cooling air which has passed through the bed of ash, is attractedin the boiler 2 from the bottom of it being this, as well known for askilled person in the technical filed, at pressure values lower to theenvironment of the casing 3.

As already mentioned, the mechanism of heat exchange between air and ashthus obtained is characterized by high thermal efficiency, thanks to thelarge ash surface available for the contact with the ambient air.

In FIGS. 2, 2A and 3, 3A typical containing lateral edges 8 flankinglongitudinally the entire transport surface 311 are also represented.

Always with reference to FIG. 1, to prevent uncontrolled entry into theboiler 2 of cooling air that escapes from the lights of the partitionedregion 4, in this embodiment are provided means of air re-circulation inthat region, preferably openable by the same forced feeding means 11. Inthe present example, these means provide a pipe for extracting air 131from the casing 3 in communication with the feeding pipe 13.

In particular—and also with reference to FIGS. 2, 2A/3, 3A—the circuit13-131 can take air from an region 15 between the casing 3 and thelateral baffles 7 (FIG. 2) or between the casing 3 and bottom faces ofthe tile 5 (FIG. 3) and send it back to the partitioned region 4 belowthe transport surface 311. The recirculated air, not having passed thebed of ash, will have a temperature close to the environment one. Alsoin this preferred embodiment example, said air outflows can beintercepted by pressure control 16 means, apt, in use, to detect bysensors a pressure difference between a first area 161 in the casing 3arranged above the transport surface 311 and a second area 162 arrangedin said casing 3 lower the transport belt 31.

In FIG. 1, these areas 161 and 162 have been depicted as arranged, forexample, at a portion of the casing 3 immediately below the combustionchamber 2. The area 162 may also coincide with the above-mentioned area15, being the pressures substantially equal in the two areas.

The pressure control means 16 are in communication with the airrecirculating means and then with the feeding forced means 11 by acontrol valve 132 selectively openable. Preferably, there is anautomatic control mean associated with the system 1 and with the means16 that, if a overpressure in the second zone 162 is detected, operatedthe valve 132 so to determine an extraction of air through the pipe 131and its return within the partitioned region 4, bringing the pressuredifference between the two areas 161 and 162 essentially at zero. Inthis way, the transfer of air outflows from the area 162/15 to the area161 is prevented.

Always with reference to FIG. 1 and now also to FIG. 5, preferably thesystem 1 also provides feeding means of cooling air to the extractionhopper 21 of the boiler 2, apt to allow a cooling of the ash held onthat said hopper when it is closed, for example, during short periods ofmaintenance of the belt 31 or any other operational need ordiscontinuous management arrangements of the system 1. Preferably suchmeans are operated by the same forced feeding means 11 and are based onfeeding means 100 even in this case with selectively adjustable air flowrate, for example by one or more valves 101.

As mentioned above, the hopper 21 provides a locking system that allowsaccumulation of the heavy ash on it. This system is formed preferably byone or more refractory valve 212 preferably servo-controlled andoperated according to a rotating closing movement.

Such feeding means of cooling air to the hopper 21 allow the cooling ofthe ash during said accumulation phase in the hopper and are preferablyoperated automatically by closing the bottom valves 212. The pipecircuit 100 feeds one or more air inlets 213 made on the bottom valves212, resulting in an homogenous distribution of the air from the bottomof the hopper 21. The entering air to the hopper 21 is of course sent ata pressure such to overcome the loss of load generated by the layer ofash accumulated, thus procuring a suitable cooling of the bed of ashpresent on the valves.

Always with reference to FIG. 1 and now also to FIG. 4, in thisconfiguration the system 1 also includes a second assembly/casingtransport belt, globally denoted by 30 and similar to the first,arranged downstream of the main belt 31 by means of interposition of anash crusher 17 and of an auxiliary cooling device 18 of tube bundle type183.

The presence of the second transport belt 30 may be advisable dependingon the amount and size of the ash. It may be associated with it forcedfeeding air means of one or more partitioned regions and eventuallyrecirculation air means similar to those already described with respectto the first transport belt 31 and preferably integrated with these. Insuch a configuration, the cooling air introduced into the area below thebelt 30 is then drawn into the boiler 2 by the pressure running existingtherein.

The crushing device 17, which may also include multiple stages offragmentation in sequence, allows increasing the ash surface availablefor the cooling, thus increasing the overall efficiency of the latter.

The auxiliary cooling device 18 provides that the ash is accumulatedwithin a volume 181 defined by walls 182 preferably metal and associatedwith these tube bundles 183, also preferably metal and constantlytraversed by a fluid at low temperature, preferably water. Still in apreferred configuration, these bundles 183 are arranged horizontally orhowever that develop in the direction substantially orthogonal to thatof a fluidizing gas flow that will be introduced shortly.

The second transport belt 30 is controlled by fed speed and transportwidth such to realize an ash head within the cooling device 18associated with it, working as ash puller from the latter. At the basisof the cooling volume 181, there is a feeding circuit of a fluidizinggas 133, preferably also with a selectively adjustable flow rate throughappropriate means such as a valve 135.

In the present example, the fluidization gas is air, and in particularthe same cooling air fed by force by the means 11 and through the valve134 and the pipe circuit 13.

The feeding of fluidizing air affects preferably the entire outerperimeter of the walls 182. The air sent in this way within the volume181 fluidizes the ash present, promoting a high number of collisions ofash particles with the surfaces of the tubes 183 cooled by the water. Inthis way an effective additional cooling of the ash is obtained all themore appreciated as much as the smaller size of the particles of thefluidised ash.

Another object of the invention is a method of extraction, cooling andrecovery of heavy energy ash as described so far in relation with thesystem 1.

The ash cooling means into the hopper and its method as described aboveand as an object of the following dependent claims could also beprotected independently from the invention as defined in claims 1 and17, and in particular independently form the expectation of air coolingmeans based on a partitioned region.

Similarly, the fluidization system of the ash in a tube bundle coolerand its method as described above and as an object of the followingdependent claims could be protected independently from the invention asdefined in claims 1 and 17, and in particular independently form theexpectation of air cooling means based on a partitioned region.

The present invention has been described so far with reference topreferred embodiments. It is intended that there may be otherembodiments which refer to the same inventive nucleus, all fallingwithin the protection of the claims set out below.

1. A cooling system for heavy ashes of the type adapted to be used inassociation with a combustion chamber, in particular for large flowsrates of ashes deriving for example from solid fossil fuel in anenergy-production unit, which system comprises: a transport belt fortransporting heavy ashes, adapted to be arranged below the combustionchamber and having a containment casing and a transport surface equippedwith openings for transit of cooling air, which transport surface isadapted to receive the ashes produced in the combustion chambersubstantially in a form of continuous bed; and cooling means for coolingthe heavy ashes received on said transport surface, which cooling meanscomprises at least one partitioned region arranged below said transportsurface and forced feeding means for a forced feeding of cooling air atsaid partitioned region, wherein said partitioned region is configuredso as to limit outflows of air fed therein, and wherein the overallarrangement is such that, in use, the cooling air fed into saidpartitioned region crosses said openings in said transport surface andthe bed of ashes received on the transport surface.
 2. The systemaccording to claim 1, wherein said partitioned region developslongitudinally along said transport surface substantially for an entireextension of the transport surface.
 3. The system according to claim 1,wherein said partitioned region is laterally delimited by one or morepairs of longitudinal baffles extending along a direction of advancementof said transport belt.
 4. The system according to claim 3, wherein oneor both of said longitudinal baffles have a door selectively openablefor a downflow of fines toward a bottom of said containment casing. 5.The system according to claim 1, wherein said partitioned region isdelimited below by a tilted-surface plate for fines recovery.
 6. Thesystem according to claim 5, wherein said plate comprises one side dooror a pair of side doors, selectively openable for the downflow of finestoward the bottom of said containment casing.
 7. The system according toclaim 1, comprising one or more pairs of side bulkheads longitudinallyextending along flanks of said transport surface, above thereto, and atsaid partitioned region so as to limit air leaks.
 8. The systemaccording to claim 1, wherein said partitioned region comprises aplurality of transverse baffles arranged transversally to said transportsurface with respect to a direction of advancement of said belt andadapted to define a substantially labyrinth-like seal for air fed insaid partitioned region.
 9. The system according to claim 1, comprisingair recirculation means for air recirculation in said partitionedregion, adapted to extract air from said containment casing andpreferably operable by said same forced feeding means.
 10. The systemaccording to claim 9, comprising pressure control means in communicationwith said air recirculation means, which control means is adapted, inuse, to detect a pressure difference between a first area in said casingarranged above said transport surface and a second area in said casingexternal to said partitioned region and arranged below said transportsurface.
 11. The system according to claim 1, comprising feeding meansfor feeding cooling air into an extraction hopper of the combustionchamber, adapted to allow a cooling of the ashes retained on said hopperwhen the hopper is closed, wherein preferably said air feeding means isoperable by said same forced feeding means.
 12. The system according toclaim 1, comprising fluidization air feeding means for feedingfluidization air into an ancillary cooling device arranged downstream ofsaid transport belt, adapted to determine a fluidized moving of theashes received therein, wherein preferably said fluidization air feedingmeans is operable by said same forced feeding means.
 13. The systemaccording to claim 12, wherein said ancillary cooling device is of tubebundle type.
 14. The system according to claim 12, comprising a crusherarranged upstream of said ancillary cooling device.
 15. The systemaccording to claim 12, comprising a second transport belt arrangeddownstream of said ancillary cooling device.
 16. The system according toclaim 9, wherein said forced feeding means, said air recirculationmeans, said air feeding means for feeding air into a hopper and saidfluidization air feeding means are connected to form a single circuitequipped with selectively operable flow adjustment valves. 17.-22.(canceled)